Conventionally, so-called sandwich panels have been used in vehicles such as automobiles, airplanes, etc., building material, casings for electric appliances, and equipment for sports and leisure.
The sandwich panel comprises two skin sheets and a core member interposed between the two skin sheets, and has a basic structure in which one skin sheet, the core member and the other skin sheet are laminated. However, a function required for such a sandwich panel varies in accordance with its application.
For instance, in case a case where an esthetic appearance is thought much of, while a high strength is not required, like an inner panel used in a bath room, a decorative sheet a surface property or an entire forming shape of which is thought much of is additionally attached to the skin sheet at the outer side and, while, in the case of an application as a structural member, strength takes priority over the esthetic appearance.
In this respect, since, in case of inner panels of vehicles such as automobiles, airplanes, a cargo floor board, a deck board, a rear parcel shelf, etc. in particular, in view of reducing the cost of fuel, since a light weight as well as a high strength are required, sandwich panels the skin sheets and the core member of which are made of resin have been adopted.
Conventionally, in such a sandwich panel, in order to attain a profile required in accordance with its application, the two skin sheets and the core member have been respectively manufactured by a cutting operation, and the manufactured two skin sheets and the core member have been assembled together and adhered to each other to complete the sandwich panel.
On the other hand, the resin sandwich panel the skin sheets and the core member of which are made of resin have been manufactured by various forming processes. Patent Publication 1 discloses a method of forming a resin sandwich panel in an extrusion forming manner.
In this forming process, the sandwich panel exhibiting good adhesion properties between the adjacent layers can be continuously manufactured by extruding each of the layers constituting the sandwich panel from a T-die and welding each of the layers in a molten state utilizing fusion heat.
Patent Publication 2 discloses a method of forming a resin sandwich panel in an injection forming manner.
In this forming process, in a sandwich panel in which a skin layer is made of a composition different from that of an inner layer, the composition for the skin layer is injected from a first cylinder up to an amount necessary for forming a thickness of the skin layer, and then, the composition for the inner layer is injected from a second cylinder at a high speed to form the sandwich panel in which the skin layer has a thin thickness.
According to the method of forming the resin sandwich panel by such an injection molding, it is feasible to form not only a molded product with a constant cross section, but also that with any profile whose cross section varies, so that there is little limitation to the shape of the molded product, as compared with the extrusion molding.
However, since, in injection molding, resin in a molten state is injected into a sealed mold space to be pressed against an inner surface of the sealed mold to be formed, it is technically difficult to form a molded product including a sealed hollow portion inside only by injection molding.
For instance, in a case where the resin sandwich panel is utilized as a cargo floor lid for an automobile, its profile has to be a complicated shape including curves with various curvatures, while, in a case where the resin sandwich panel is utilized as a structural member, an inner structure with its stiffness sufficient to withstand a local external load is required.
In this respect, as a first aspect, it is desired to realize a core member for a sandwich panel and a method of forming said core member for the sandwich panel which are capable of attaining a desired profile, a desired surface shape, and a desired inner structure in accordance with an application of the sandwich panel.
In a case where the resin sandwich panel is utilized as a cargo floor lid for an automobile, not only a good external esthetic appearance, but also a high stiffness (a high flexural rigidity, in particular) to withstand the weight of luggage are required, since the cargo floor lid is used in such a way that heavy luggage is rested thereon, while on the other hand, a light weight thereof is required so as to reduce the fuel cost. Such being the case, it is crucial to solve a difficult technical problem, that is, compatibility of high stiffness with light weight. For this reason, as the resin sandwich panel for such an application, a rigid resin with a high Young's modulus has been adopted for the skin sheet, while, in the core member, a thickness of the core member has been increased to enhance the section modulus by enlarging the distance between the two skin sheets, while, foamed material has been adopted for the core member itself, or a hollow portion has been provided inside thereof, or a plurality of recesses have been provided on its surface in order to reduce the weight of the sandwich panel.
Recently, much attention has been paid to a resin core member with a so-called honeycomb structure for the resin sandwich panel with a high stiffness and a light weight.
The resin core member with such a honeycomb structure is roughly divided into two types, one being that in which a plurality of recesses each opening of which on the one surface of the core member is shaped to be a regular hexagon are provided on one surface so as to protrude at a side of the other surface, the other being that in which a plurality of perforated holes each opening of which is shaped to be a regular hexagon are provided on the surface of the core member.
According to such a resin core member with the honeycomb structure, the void volume can be secured to reduce its weight, since it is possible to densely arrange the plurality of recesses or perforated holes on the surface of the core member in the most efficient manner.
Patent document 3 discloses a sandwich panel including a resin core member with a number of truncated conical recesses formed by an extruding forming process using a stepped roll.
This sandwich panel comprises two skin sheets made of resin and a core member made of thermoplastic resin interposed between two skin sheets, and the core member comprises a pair of plates made of thermoplastic resin on an outer surface of each of which a plurality of truncated conical recesses each of which inwardly tapers in a converging manner are provided. Each of the plurality of truncated conical recesses is formed by a plurality of protrusions provided on the surface of the stepped roll and protrudes at an inner surface side to form an abutting planar portion at its most convergent portion. The core member is formed by abutting the butting planar portion of each of the recesses of one of the pair of plates against that of the corresponding recess of the other of the pair of plates through a clamping force caused by the stepped roll.
According to the above sandwich panel, its weight can be decreased by heightening a void volume by means of the plurality of recesses.
However, a sufficient flexural rigidity as an entire sandwich panel cannot be attained, since adhesion properties between the pair of plates constituting the core member are poor.
More specifically, a pair of resin plates are caused to pass between rollers including the stepped roller to be adhered to each other by a pressing force being applied thereto, so that sufficient adhesion properties are not obtained due to the facts that the adhesion between the pair of resin plates is not a surface-to-surface adhesion, but close to a point-to-point adhesion, and that the time period for adhering the pair of plates is short.
In this respect, according to the forming process using mold(s), although each of the plurality of recesses have to possess an inwardly converging taper angle in view of a limitation that the forming is carried out using the mold(s), such a technical problem is not caused, because a sufficient adhering time period can be obtained while the molds are clamped.
Patent document 4 discloses a method of manufacturing a resin sandwich panel by sheet forming (compression molding), which is one of the forming processes using molds.
More specifically, the skin sheet and the core member (and the decoration member as the occasion demands) are disposed between the pair of split molds, and then, the split molds are closed, whereby the skin sheet and the core member within the closed molds are pressurized to be formed, while the skin sheet and the core member are welded. In this respect, it is possible to form the sandwich panel at one time.
However, in order to form or weld by closing or clamping the molds, each of the skin sheet and the core member has to be in a molten state, so that, in a case where the skin sheet is fed from a raw material sheet roll as a continuous sheet, for instance, it is necessary to heat it again with an infrared heater, etc. before it is positioned between the pair of split molds. The following technical problems can be caused due to such a reheating.
Firstly, in the sandwich panel formed by the sheet forming process, it is difficult to attain a complicated outer shape and a complicated pattern on its surface. More specifically, it is difficult to bring the inside of the sheet into a sufficient molten state by such a reheating which heats the sheet from its outer surface. If the time period for heating the sheet is lengthened in order to make the inside of the sheet a sufficiently molten state, the sheet is caused to sag under its own weight, which makes the forming process itself difficult. Even if the sheet is formed, it is not technically feasible to attain a complicated outer shape or an embossed pattern, etc. on its surface, since characteristics in which the sheet is formed along the cavity of the mold becomes worse.
Secondly, in the sandwich panel formed by the sheet forming process, it is difficult to obtain a sandwich panel with sufficient strength in a case where it is used as a structural member. More specifically, since the welding strength of an outer peripheral parting line PL which is a welded portion between the skin sheets, or the welding strength of a welded portion between the skin sheet and the core member becomes worse due to a bad influence of the above-described reheating on the molding properties, the strength of the entire sandwich panel becomes deteriorated.
In view of these, as a second aspect, it is desired to realize a method of producing the sandwich panel which is capable of producing the sandwich panel with any outer shape or any pattern on its surface exhibiting a sufficient strength without a need to reheat the elements such as the skin sheets, the core members, etc.
In this connection, according to a blow molding or a vacuum forming process which is one of the forming processes using mold(s), it is feasible to form a molten resin as it is, so that efficient forming can be realized by omitting the process in which the molded product as a molding is reheated to be formed as a post-forming.
However, if the sandwich panel is formed by blow molding or vacuum forming, a peculiar technical problem associated with the blow-up ratio is caused.
More specifically, as to the flexural rigidity of the entire sandwich panel, on the condition that the adhesion properties between the core member and each of the pair of the skin sheets is secured, the larger the thickness of the core member becomes, the more the flexural rigidity of the entire sandwich panel can be improved due to the increase of the section modulus of the pair of the skin sheets.
However, on the other hand, the larger the thickness of the core member becomes, the bigger the blow-up ratio becomes, so that a phenomenon in which a peripheral edge portion of an opening of each of the plurality of recesses on the core member becomes dull, whereby the flatness is so reduced that the adhesion area on the surface of the core member is substantially lost, and as a result, it becomes difficult to secure sufficient adhesion properties between the core member and the skin sheets. On the other hand, in a case where the tapering angle of each of the plurality of recesses is constant, the larger the thickness of the core member becomes, the bigger the aperture rate on the surface of the core member by the plurality of recesses becomes, whereby the adhesion area is lost, while the void volume caused by the plurality of recesses is increased to decrease the weight of the sandwich panel.
Such being the case, in a case where the sandwich panel is formed by blow molding or vacuum forming, the balance between the thickness of the core member and the tapering angle of each of the recesses is important in order to decrease the weight of the sandwich panel, while at the same time to attain high stiffness.
On the other hand, in a case where a bending load is applied to the sandwich panel, the maximum bending stress is generated on the lower and upper skin sheets. Even if the thickness of the core member is secured and sufficient adhesion properties between the core member and the skin sheet are maintained, the core member itself can be destroyed or fractured. In this respect, it is also necessary to secure a structural integrity of the core member itself.
In this respect, as a third aspect, it is desired to realize a sandwich panel with a light weight and a high stiffness by having the core member include a hollow portion and yet secure the strength, while at the same time by having the skin sheets possess the strength higher than that of the core member, and secure the sufficient adhesion properties between the core member and the skin sheets.
In this connection, patent publication 5 discloses a structural plate made of synthetic resin produced by blow molding. More specifically, the structural plate consists of a plate body made of thermoplastic resin produced by blow molding on a face side and an underside of which a plurality of recesses are formed, respectively, in such a way that the bottom portion of each of the plurality of recesses on the face side and the corresponding bottom portion of each of the plurality of recesses on the underside are disposed back to back to be opposite to each other.
According to such a structural plate, the weight can be decreased while at the same time its compressive and tensile strength can be improved.
However, patent publication 5 only discloses a structural plate made of synthetic resin, and does not discloses a sandwich panel constituted by a core member like this structural plate and the skin sheets fixed on the face side and the underside of the core member, respectively, much less the flexural rigidity as a sandwich panel based on the stiff adhesion properties between the skin sheet and the core member.
Patent publication 1: Japanese Patent Laid-open Publication SH055-67444
Patent publication 2: Japanese Patent Laid-open Publication 2005-132016
Patent publication 3: Japanese Patent Laid-open Publication 2006-103027
Patent publication 4: Japanese Patent Laid-open Publication HEI07-171877
Patent publication 5: Japanese Patent Laid-open Publication 2008-247003